Project Summary The global objective of this research is to elucidate the mechanisms underlying tissue homeostasis and regeneration in mammalian skin and to understand how this process goes awry in human disorders, including cancers. Central to achieving this goal is the purification and characterization of the differerent stem cell (SC) populations within skin and determination of their relative contributions to tissue homeostasis and wound- repair. Past AR050452 research led to purification of hair follicle (HF) bulge cells, and established them as self- renewing SCs that function in hair cycling and wound-repair. These HF-SCs also displayed more robust self- renewing and broader tissue regenerative potential than interfollicular epidermal (IFE)-SCs. Several differences might account for this. One is that like human IFE-SCs, mouse HF-SCs exist in a quiescent state for prolonged periods. Another is that they uniquely express transcription factors such as Nfatc1, which when absent, causes HF-SCs to lose quiescence and cycle HFs continuously. Past AR050452 research has set the foundations to tackle the following key questions: (1). Is HF-SC quiescence critical to their long-term maintenance? What happens when HFs cycle continuously? Do they expend their SCs over time? Can they heal wounds faster? (2) What are the molecular mechanisms governing the resting phase of the HF-SC niche? How does Nfatc1 and its associated transcriptional regulators function to maintain HF-SC quiescence? What are its direct targets and how do they regulate the balance between quiescence and long-term self-renewal? (3) What molecular changes define the difference between stemness and the transit amplifying state? Do stem cell progeny influence this transition and if so how?(4) What are the relative contributions of IFE and HF SCs to wound-repair? Does this differ in superficial vs deep wounds? In young vs adult mice? (5) How do IFE and HF SCs alter gene expression in response to injury? How fast are these changes and what controls the process? To answer these questions, we'll use FACS, RNA-seq, ChIP-seq, conditional gene knockout and RNAi screens in vivo and employ these methods to explore skin stem cells in their native, mutant and wound-induced environments.